The present invention relates to neutral blue zirconium-vanadium pigments which are distinguished over generic previously known pigments by distinctly improved color values in the CIE-L*a*b* color system. Further subject matter of the present invention relates to a method of producing such neutral blue zirconium-vanadium pigments.
Glaze-stable coloring pigments based on zirconium silicate doped with vanadium have long been known (U.S. Pat. No. 2,441,447). Such coloring substances, frequently designated as zirconium-vanadium blue, are produced by means of annealing a mixture of zirconium compounds, silicon compounds, and vanadium compounds which are capable of forming ZrO.sub.2, SiO.sub.2 and V.sub.2 O.sub.5. The powder mixture, which is to be annealed at 650.degree. to 1200.degree. C., generally contains one or several mineralizers from the series of alkali metal oxides, hydroxides, carbonates and fluorides. The color values, measured in the CIE-L*a*b* color system according to DIN 5033 (see part 3, 01/80) using a 5% by weight staining or dyeing in a transparent glaze are in the range of L*=37 to 45, a*=-5 to -12 and b*=-25 to -35 for pigments produced under very favorable conditions. Even if the literature discusses only zirconium-vanadium blue, it should not be overlooked that this concerns a Turkey blue coloring substance (i.e., one with a relatively large green component characterized by the strongly negative a* value). A neutral blue which is characterized by an a* value about 0, a b* value more negative than -20 and an L* value of less than 50 was not able to be obtained within the framework of the disclosure of U.S. Pat. No. 2,441,447, even with variation of the production conditions. There is great interest in so-called corner coloring substances, in this case a neutral blue pigment with an a* value near 0, because such coloring substances are well suited for the production of pigment mixtures for the purposeful adjustment of quite specific color locations and can thus replace cobalt-containing coloring substances as required.
GB 1,447,276 teaches greenish-blue to bluish-violet or grayish-violet coloring substances based on zirconium silicate doped with vanadium. The color shift of the known greenish-blue to a more reddish tone is obtained in the method described in this document in that the mixture to be annealed additionally contains a small amount of an inorganic phosphate or of an organic phosphoric acid ester. Thermally dissociated zirconium silicate is used as the exclusive source for ZrO.sub.2 and SiO.sub.2. It was determined that the neutral blue or slightly reddishly tinged blue coloring substances obtained according to this method also contain phosphorus in addition to vanadium in the zirconium silicate host lattice. The neutral blue, i.e., phosphorus-containing zirconium-vanadium pigments produced according to GB 1,447,276, are characterized by a low color intensity (L* value around or above 60) and an insufficient color saturation which is brought about in particular by low b* values from -5 to -19.
A further disadvantage of the neutral blue pigments produced according to the method of GB 1,447,276 is the fact that they can not be produced in reproducible quality (i.e., with low color intervals measured according to the CIELAB formula in accordance with DIN 6174 (Jan. 1979)). This disadvantage is surprising at first because especially favorable prerequisites for the formation of host lattice pigments were expected from thermally dissociated zirconium silicate since an especially intimate mixture of the two components SiO.sub.2 and ZrO.sub.2 is present in this instance. One of the causes for this disadvantage is suspected in the optionally changing content of non-dissociated zirconium silicate in the thermally dissociated zirconium silicate. The new formation of ZrSiO.sub.4 from SiO.sub.2 and ZrO.sub.2 is obviously seeded by thermally non-dissociated zirconium silicate so that the required doping with vanadium and phosphorus remains insufficient, which brings about a diminution of the color intensity.